Heretofore, the production of silicon carbide whiskers has always been achieved by batch processing methods. In one known process, rice hulls, a throw-away byproduct of rice refining, are known to contain both silicon dioxide and carbon in sufficient amounts to produce silicon carbide as taught in U.S. Pat. No. 3,754,076. Rice hulls are generally batch heated in an induction furnace to temperatures in the general range of 1600.degree. C. to 2000.degree. C. for several hours. During this time, the silicon and carbon combine to form a cake of both silicon carbide particles and silicon carbide whiskers. The whiskers are removed and separated from the cake by well known methods, such as froth flotation, as described in U.S. Pat. No. 4,293,099.
Silicon carbide whiskers can also be made by the batch heating of organic fibers blended with silica at temperatures of 1400.degree. C. to 1700.degree. C. as shown in U.S. Pat. No. 4,284,612.
These whiskers find important uses in the reinforcement of metals, plastics and ceramics.
It has been known that silicon carbide particles (as distinguished from whiskers) for use as abrasives can be produced in a continuous process. Canadian Pat. No. 544,597 shows a process in which mixtures of coke, sand and salt (the latter apparently as a catalyst) are moved through a furnace. The mass of materials moves about and expands substantially as the coke and sand react to form blocky SiC crystals. Exhaust gases evolved from the reaction are removed at the inlet end of the furnace. However, attempts to produce whiskers in continuous processes have not heretofore been successful. The reaction does not easily lend itself to continuous production methods, because continuous feeding of the coked rice hulls or other raw materials agitates the reactants, thus inhibiting the growth of the whiskers.
Attempts have been made by the present inventors to feed these materials on a continuous batch basis. Cylindrical graphite containers filled with coked rice hulls were pushed in a sequential manner through a heating zone. These graphite containers were used to maintain the feed materials in an unagitated state, while continuously traveling through a conversion furnace.
This initial process failed after several days of continuous operation. Analysis of the problem revealed that gaseous impurities emitted during the conversion process were condensing and forming glass-like deposits on the furnace and exit walls in areas whose temperatures generally fell below 1500.degree. C. These glass-like deposits prevented the movement of the graphite containers, i.e., the deposits formed a friction surface against which the containers could not be fed.
Further, a problem was noted with moisture in the feed materials. Moisture present at temperatures necessary for whisker formation has the ability to erode furnace wall.